CNIDARIA 127 



sented in the ontogeny of Pelagia, for example, by the stage 

 of the gastrnla invaginata, i.e., a ciliated, ovoid, free- 

 swimming form, in which an archenteron opening to the 

 outside world by means of the prostoma was developed by 

 an invagination at the posterior end. 



It can easily be explained how an ovoid blastula-like heteropolar an- 

 cestral form happened to develop the earliest beginnings of the archen- 

 teric invagination at the posterior pole of its body. In the case of 

 monaxial, heteropolar blastular larvfe which are allowed to swim through 

 water containing particles of carmine, it can be seen that these particles 

 are repulsed at the anterior and lateral parts of the body by the move- 

 ments of the larvae, whereas they are crowded together at the posterior 

 pole. Here accordingly was a favourable place for the reception of 

 particles of food, and by a flattening or shallow invagination of the 

 posterior pole these favourable conditions were increased. The archen- 

 teron therefore in its earliest beginnings was a pit in which to catch 

 particles of food. 



If we incline to the view that the hypothetical ancestral 

 form of the Cnidaria was similar to the gastrula invaginata, 

 then in most Cnidaria we must assume a secondary change 

 in the ontogeny, for the typical lariml form of the Cnidaria 

 is the planula, a form in which we can recognize a ciliated 

 ectoderm and a more or less compact entodei'mal mass within. 

 The taking of food is here suppressed. This form serves 

 exclusively for locomotion and the consequent dissemination 

 of the species over a larger territory. In attached forms 

 such larval conditions are of great importance for the pre- 

 servation and distribution of the species. In the interest 

 of this function, the archenteric cavity appears to have 

 degenerated in the planula. 



It is probable that the transition from the free-swimming 

 gastrula-like ancestral form to the attached polypoid foi-m 

 was brought about by means of an interpolated cj-eeping 

 stage, which would be recalled by the creeping planula of 

 many existing forms (e.gr Lucernaria). 



Literature. 



CxiDAKiA IN General. 



1. Agassiz, a. Illustrated Catalogue of the Mus. Comp. Zool. Harvard 

 Coll. No. II. North American Acalephas. Cambridge, U.S. 1805. 



